1. Field of the Invention
This invention relates generally to semiconductor solar energy devices and fabrication methods therefor and, more particularly, to semiconductor solar energy devices of the PN diode type having an anti-reflective coating on one side thereof and a textured pyramid shaped silicon surface beneath the anti-reflective coating, and fabrication methods for making such devices.
2. Description of the Prior Art
Prior art techniques and method steps for making semiconductor solar energy devices were generally very complex and therefore very costly. In the past, semiconductor solar energy devices were made using PN diode type structures wherein an anti-reflective coating of a dielectric such as tantalum oxide or silicon monoxide was used on the sunlight-striking side of the solar energy device. These two materials are not commonly used in silicon devices and present manufacturing problems.
In this prior art type of semiconductor solar energy device, ohmic contacts were formed to the backside thereof and also to the front side of the device. The ohmic contacts to the front side of the device which was on the same side as the anti-reflective coating generally provided a problem because these metal contacts very often shorted through the underlying diffused regions into the semiconductor region of opposite type conductivity located beneath the diffused region on which the contacts were located. It was generally undesirable to provide a diffused region on the sunlight striking side of the device which would have a thickness greater than 0.3 microns. The reason for this is that a shallow PN junction is necessary for optimum collection of generated electron-hole pairs created by photon bombardment when subjected to solar energy. Consequently, during sintering of the metal ohmic contacts on the sunlight-striking side of the prior art semiconductor solar energy devices punch through or shorting problems developed in fabricating these types of devices because of the presence of this thin diffused region on the sunlight striking side of the device and the metal penetration during sintering.
Another problem associated with the prior art semiconductor solar energy devices is that the metal contacts that were applied to the semiconductor solar energy device required several costly or low manufacturing yield steps which made the prior art devices either unreliable or more expensive to manufacture.
The utilization and desirability of a rough solar cell surface, consisting of a uniform distribution of minute pyramids, to increase solar cell efficiency has been demonstrated. This rough or textured surface causes all the light that is reflected from the first impingement on the rough solar cell surface to strike the solar cell at least a second time (assuming initial normal incidence). This second impingement increases the amount of light absorbed in the solar cell, improving cell efficiency. Such a solar cell has recently been exhibited by COMSAT.
There are other advantageous features of such a textured surface on a solar cell. The major effect is that, since light is refracted into the silicon at an angle to the normal of the overall solar cell plane, more light is absorbed within a given thickness of silicon than would occur with normally incident sunlight on a smooth-surfaced solar cell.
However, prior art etchants used to create such a textured pyramid shaped silicon surface were generally unstable or produced variable results, and thus are undesirable for use in manufacturing large quantities of semiconductor solar energy devices. Furthermore, prior art silicon solar cells did not use a textured pyramid shaped silicon surface in combination with other features such as an anti-reflective coating containing dielectric materials that have been used in semiconductor processing, ohmic metal contacts that will not short through a thin doped region of the solar energy device, etc.
An optimum process for solar cells should have the following features:
1. Minimum number of total steps. PA1 2. Minimum number of photoresist steps. PA1 3. Dopant concentration a maximum at the semiconductor surface, monotonically decreasing into the bulk. PA1 4. Heavier dopant concentration below metallization areas for improved ohmic contact. PA1 5. Greater junction depth below metallization than active areas to reduce the possibility of metal punch-through, while retaining cell sensitivity. PA1 6. Have an anti-reflective coating. PA1 7. Have a textured pyramid shaped silicon surface. PA1 8. Minimize wafer exposure time to high temperatures.
A need existed for providing a semiconductor solar energy device and process therefor that would overcome these disadvantages of prior art devices and processes and which would have the above identified process features to permit large quantities of these devices to be manufactured at relatively lower cost.